(624b) Investigating the Effect of Binuclear Cu-Oxo Structure Heterogeneity in Cu-CHA Zeolites on Stoichiometric Partial Methane Oxidation Reactions

Partial methane oxidation to methanol (PMO), one of several reduction oxidation reactions catalyzed by Cu-ion exchanged aluminosilicate zeolites, is performed in a stoichiometric cycle in which each reactant is introduced sequentially. Binuclear Cu species (Cu₂O_y, y=1,2) formed upon high temperature O₂ treatment were identified as likely PMO-active sites in the CHA framework. Prior work by Brezicki et al. in Cu-MOR and Cu-MFI attributed different Cu site distributions (Cu₂O₂ versus Cu₂O), reportedly altered by varying the co-cation (Na⁺ versus H⁺), to changes in CH₃OH yields and selectivity. A separate study by Rhoda et al. also reported CH₄ activation barriers depend on the geometry of CuOCu sites. These results suggest a variety of Cu active sites form during PMO, convoluting the establishment of relationships between site structure and PMO yield and selectivity. Here, we measure individual product yields (CH₃OH, CO, and CO₂) to calculate total oxygenate yield as a reporter of oxygen content in Cu species and investigate how O₂ activation time influences interconversion among Cu species in Cu-CHA. Stoichiometric PMO yields were measured on Cu-CHA samples of varying Cu density (Cu/Al=0.055–0.22) after O₂ treatments (21 kPa O₂, 723 K) of different durations (0.5–30 hours). Systematic trends in individual product and total oxygenate yields were observed with O₂ activation time and were dependent on Cu density, used to support a reaction scheme for the formation and interconversion of binuclear Cu species as a function of O₂ time. Additional experiments with varying O₂ activation conditions (e.g., subsequent purge in He, or O₂ activation with co-fed H₂O) were performed to further probe mechanisms of binuclear Cu site evolution. Altogether, this study examines the mechanisms of Cu active site formation and the effect of site structure on PMO yield and selectivity in Cu-CHA zeolites comprising a heterogeneous pool of binuclear Cu sites.